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Strange Engine Noise

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Mercgurr

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I have a 3 month new 2021 Merc 50 HP 4-stroke fuel injected outboard. Recently it started making a strange intermittent sound. Now the sound is continuous. It sounds like metal scrapping metal. It only happens when the prop is put in forward or reverse and pushing water, no noise in neutral. I removed the cowling and the flywheel cover to be sure there was no rubbing -- and observed the starter was not rubbing the flywheel. I'm going to attempt to attach a video/audio link that should be available by Sun Feb 14 12 AM. Appreciate any and all opinions.
 
Thanks guys for your replies. The dealer did change the oil in the lower unit just to see if it would make a difference which it didn't. Still heard the noise. And the lower unit oil was not milky nor did it have any metal shaving in it.
 
No, but it's not leaking oil. I have a tester so I could pressure test the LU. What are you thinking the test might prove? The shop foreman wants to meet me at the nearest boat ramp to my house so he can hear the noise in the lake and try for a diagnosis. Then he'll tow the boat back to the dealer to attempt a fix. Lots of rain now so I'll probably meet him Tuesday.
 
Update 2-16-21 -- Met mechanic at a boat ramp and first thing he did was remove the prop. He got in the boat and I backed it down the ramp into the water. He ran the motor in forward, reverse, and neutral, there was no noise. I pulled the boat out and he replaced the hub in the prop. The hub showed some damage. We again ran test with prop on and heard the noise. He towed the boat to the dealer/shop. Later the mechanic called and said the noise was the prop rubbing against the housing. There was a sign of friction on the tip of the prop blades but I thought that was maybe when I got in some sand. So now they want to replace the complete lower unit. Problem is they are on back order. I suggested they just grind away the housing metal to give the prop clearance but they said that would void out my warranty. I'll give them a couple of days to see what is going to happen and then I'll go to dealer and talk options. I'm wondering if a SS prop would scrape the housing?
 
Does he mean it's rubbing on the inward side of the prop, not the blade tips? I Don't think prop material will matter. I would suggest trying a new prop before grinding anything, maybe it was a bad casting or was damaged? I figured hitting something was already ruled out when it was out of the water so I didn't bother asking. I would check the prop shaft runout before ordering anything unless the LU is under warranty then I'd just replace it.
 
As you stand behind the motor and looking at the prop there is some friction showing on the prop tips. About one-half inch of the tips. When I first showed it to a mechanic he thought I may have run the prop in some sand. That seemed plausible to me. Because two other props were tried with the same noise it appears the housing is at fault. I'm mad at myself because i didn't look closer to see what the prop was scrapping against. I guess had I kept running the motor eventually the black paint would have worn off the prop showing aluminum metal and then I would have seen the prop was scrapping the housing. The motor is under warranty. There is no indication anywhere on the LU or the prop that the motor ever hit any submerged object. The mechanic says he believes Mercury will want to replace the LU.
 
This is a drawing that I colored to reflect how the prop looks. Slight discoloration on the tips where it was rubbing.
Prop A.jpg
 

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Docksidemarineservices said:
So the prop is hitting the skeg? I would look at the hub and make sure all parts are correct and in place... Especially the thrust washer.
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Good advice. Thanks. I'm not sure what part of the LU the prop is rubbing against. Everything is as it came from the factory. So why did the prop start rubbing after about 10 hours of use? No dents on the LU nor on the prop. The boat is now in the shop. I'm going over tomorrow to talk options with them.
 
Been reading this and wondering.......why don't they check clearance while turning the prop/props by hand in neutral? There needs to be adequate clearance when pushing the prop shaft both fore and aft. If the prop is rubbing that seriously, the condition causing it should be obvious. Could these marks be misleading and the noise is from the lower unit itself when prop is pushing water.....thrust bearing?
 
timguy said:
Been reading this and wondering.......why don't they check clearance while turning the prop/props by hand in neutral? There needs to be adequate clearance when pushing the prop shaft both fore and aft. If the prop is rubbing that seriously, the condition causing it should be obvious. Could these marks be misleading and the noise is from the lower unit itself when prop is pushing water.....thrust bearing?
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There is no rubbing when turning by hand, best I remember (I don't have the boat at home now). But turning by hand is just a slow turn. I don't think it's the thrust bearing because the noise is heard when the motor is in both forward and reverse. Doesn't forward and reverse put a different thrust direction on the prop/bearing/shaft? Something is rubbing the prop. All along I have said the sound is like a car's fan hitting the radiator.
 
More confusion -- So the dealer said a new lower unit is going to be sent by Mercury -- they are on back order -- you know how long that takes. So I drove over to bring my boat home and put it under a carport. I decided i could use my trolling motor to fish. At the dealer the mechanic said he would re-install the prop which they had removed. He then showed me where the rubbing was taken place -- not the blades -- but just inside the lower unit where the prop slides in about 3/8". I misunderstood when we spoke on the phone and he explained it to me. I brought the boat home and then decided to remove the prop and grease the rubbing area inside the LU. Well it was then that I examined the prop and notice the rubbing on the prop on the collar that inserts inside the LU is only on 1/3 of it. So if the LU shows rubbing at 360 degrees and the prop shows rubbing at only 1/3 of the collar that indicates to me the prop is out of round. I'm going to call the dealer and discuss my findings with the mechanic. Stay tuned . . .
 
Mercgurr said:
More confusion -- So the dealer said a new lower unit is going to be sent by Mercury -- they are on back order -- you know how long that takes. So I drove over to bring my boat home and put it under a carport. I decided i could use my trolling motor to fish. At the dealer the mechanic said he would re-install the prop which they had removed. He then showed me where the rubbing was taken place -- not the blades -- but just inside the lower unit where the prop slides in about 3/8". I misunderstood when we spoke on the phone and he explained it to me. I brought the boat home and then decided to remove the prop and grease the rubbing area inside the LU. Well it was then that I examined the prop and notice the rubbing on the prop on the collar that inserts inside the LU is only on 1/3 of it. So if the LU shows rubbing at 360 degrees and the prop shows rubbing at only 1/3 of the collar that indicates to me the prop is out of round. I'm going to call the dealer and discuss my findings with the mechanic. Stay tuned . . .
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That's where I thought it might have been rubbing.. The gap between the blade tips and the anti-cavitation plate is not a huge gap, but nothing that would be making contact without having had serious damage to the prop shaft (or the wrong prop). I believe on these newer outboards I have seen a plastic collar of sorts on the inner portion of the LU where the prop rides that has been broken or become distorted, causing a rubbing point, but nothing metal to metal sounding like in your video.
 
Props are not perfect in this area. I have seen this before. Your lower unit is probably just fine. There is a shoulder or "step down" where prop hub enters the lower unit. You could grind this down and create adequate clearance. Not sure I would go for the lower unit replacement. It will probably do the same thing as the original unit. This is likely a minor engineering miscalculation regarding clearance. A different prop manufacturer may have already adjusted for this......could try a stainless prop, they are thinner and perhaps more precisely sized in the hub area. They are also a little more efficient because the blades are thinner, since stainless is 6 times stronger than aluminum and most of it's alloys that are used for propellers.
 
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Well I have been all over the place explaining my problem and now I have more info. I purchased a used prop, slightly different circumference and pitch. When I torqued it to the shaft it didn't go forward enough for the step-down to insert into the lower unit. There was a slight gap between prop and LU. I thought that would solve the problem. I took the boat out this morning and the motor was still screeching and the boat was vibrating. In fact the noise was louder. No doubt now that the problem is in the LU. I returned home and called the dealer. I told the general manager fishing was my passion and I need to hear some options from him to get me back on the water without having to wait for an unknown time for a replacement LU. He said he would call me back on Tuesday with options and info. Stay tuned . . .
 
I arrived at the dealer at 8:55 AM and was heading home at 9:55 AM with new LU installed and tested. I will be taking the boat fishing tomorrow and test the motor under load. It does look like it could be a thrust bearing. The dealer said they would not tear down the unit to see what was the problem, they are just going to ship it back to Mercury. Stay tuned for the final test tomorrow . . .
 
Great. They might not be authorized to inspect in this situation, but should get a report back from the factory. Good fishing, my friend.
 
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Thanks. I told the dealer I would call and let them know if the problem was solved, or not. If all goes well I'll check back with the dealer in a week to see if they received a report from the factory.
 
Launched the boat and ran the motor at various RPM's, both forward and reverse, and I'm happy to report it was quiet and no more screeching sound. The problem was definitely in the LU. Also no more vibration in the deck of the boat. I called the dealer to let the manager and mechanics know the problem was fixed. Appears timguy had the correct diagnosis. Congratulations.
 
Great, catch any fish?......now you can "sneak up on em". Be nice to get a report from the factory to see what it is for sure.
 
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